Optionally embed NOTICE files in apks.

If embed_notices or ALWAYS_EMBED_NOTICES is set, collect NOTICE files
from all dependencies of the android_app, merge them with the app's own
one (if exists), transform it to HTML, gzip it, and put it as an asset
in the final APK output.

Bug: 135460391
Test: app_test.go
Change-Id: I52d92e2fd19b3f5f396100424665c5cc344190d8
diff --git a/Android.bp b/Android.bp
index fff17ef..fe3a390 100644
--- a/Android.bp
+++ b/Android.bp
@@ -55,6 +55,7 @@
         "android/mutator.go",
         "android/namespace.go",
         "android/neverallow.go",
+        "android/notices.go",
         "android/onceper.go",
         "android/override_module.go",
         "android/package_ctx.go",